The antibody-facilitated (AF) bone marrow (BM) chimera is an experimental model of BM transplantation that we have developed that does not utilize irradiation for host conditioning. The induction protocol involves only the injection of donor BM cells and anti-host monoclonal antibody (mAb) directed against class I or class II antigens of the major histocompatibility complex (MHC). The recipient displays complete hemopoietic takeover, immunocompetence and host-specific tolerance. This project will determine the mechanisms of action and host conditioning of anti-MHC mAbs in vivo as well as to address the mechanisms underlying the induction and maintenance of transplantation tolerance. Recipient mice will receive cobra venom factor to determine if antibody induction of hempoietic engraftment is complement independent. The importance of immunoglobulin class and Fc fragment will be examined in the ability of anti-host class I and class II IgG2a and IgM mAbs or F(ab')2 IgG2a mAbs to produce AF chimeras. The modulation of class I and class II surface antigen expression on spleen cell populations will be determined from anti-MHC mAb-treated mice by flow cytofluorometry. To determine host conditioning, anti-MHC mAbs will be administered in vivo and the recipients will be examined for host resistance to in vivo colonization of radiolabeled hemopoietic precursors and the recipient spleen cells will be tested for natural killer activity, interleukin-2 production, mixed-leukocyte reactivity and cell-mediated lympholysis in vitro. Manipulation of the host by stimulation with aduvant, IL-2 and interferon or by adding back recipient-type cells will permit a clearer understanding of graft resistance. The stem cell and stromal cell targets of mAb therapy will be determined using in vitro methycellulose colonies and long-term BM cultures. The mechanisms underlying the induction and maintenance of tolerance in AF chimeras will be evaluated by both cytotoxic and helper T-cell precursor frequencies during and after the establishment of hemopoietic chimerism. Active suppressors will be determined by the shape of the curve in limiting-dilution analysis and by cell-mixing experiments. In addition, the presence of anti-idiotypic suppression and antibodies will also be examined. This project is planned to use this AF chimera model in which long-term stable chimerism is achieved to determine how the hemopoietic and immunologic compartments are affected by anti-MHC mAbs in the establishment of long-term stable chimerism.